Medium For Resin Particles Containing Fluorescent Dye

ABSTRACT

An object of the present invention is to provide a medium which is capable of inhibiting precipitation and/or aggregation of fluorescent dye-containing resin particles and enables to use the fluorescent dye-containing resin particles for staining after a long-term storage without having to perform complicated operations. The present invention provides a medium for storing fluorescent dye-containing resin particles, wherein, in a particle-containing liquid obtained by adding fluorescent dye-containing resin particles to the medium, the rate of change in the backscatter intensity (transmitted light) at the center of the height of the particle-containing liquid left to stand for 24 hours after the addition is not less than −1% based on the particle-containing liquid immediately after the addition.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/305,416 filed Oct. 20, 2016, which is a U.S. National Stage ofInternational Application No. PCT/JP2015/061567 filed Apr. 15, 2015,which claims priority of Japanese application no. 2014-089287 filed Apr.23, 2014, the entire content of all of which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a medium used for storing fluorescentdye-containing resin particles.

BACKGROUND ART

In recent years, fluorescent dye-containing resin particles have begunto be used as fluorescent labels in the field of biology. Fluorescentdye-containing resin particles are particles having a structure in whicha fluorescent dye is encapsulated by an appropriate resin particle. Asfluorescent dye-containing resin particles, those in the form of acomplex with a functional group or molecule that is capable of bindingto a biological substance such as an antibody may also be used inapplications such as immunostaining.

When such fluorescent dye-containing resin particles are used, they arenot always used immediately after the production and may be stored for acertain period until use. In that case, the fluorescent dye-containingresin particles are often stored in a state of being diluted in a mediumso that the functions as a fluorescent label can be maintained.

As a medium for storing fluorescent dye-containing resin particles, anappropriate buffer containing a small amount of a blocking agent or asurfactant-containing liquid is used in many cases so that aggregationand the like of the fluorescent dye-containing resin particles can beinhibited as much as possible. For example, Patent Document 1 describesthe use of 1% BSA/PBS buffer as a medium for storing fluorescentdye-containing resin particles.

However, even in those cases where such a conventional medium is used,when fluorescent dye-containing resin particles stored for a long periodare directly used for various staining processes such as immunostaining,coarse aggregates are generated in the resulting stained cellular tissueimage, which may interfere with correctly counting the number of brightspots. In order to avoid such a situation, conventionally, thosefluorescent dye-containing resin particles that have been stored in astate of being diluted with a medium over a long time are required to besubjected to pretreatments such as solvent substitution, which isperformed by repeating appropriate times the operations ofcentrifugation, supernatant removal, dilution with a staining solventand redispersion by ultrasonication, and subsequent filtering treatment,prior to being used for staining; therefore, there is a problem ofhaving to perform complicated operations.

CITATION LIST Patent Document

-   Patent Document 1: WO2012/029342

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When fluorescent dye-containing resin particles are stored for a longperiod using a conventional medium, coarse aggregates are oftengenerated upon staining cellular tissues with the stored fluorescentdye-containing resin particles. Such coarse aggregates can usually beobserved as aggregates of a size equivalent to a 2.5 to 5-μm square orlarger and may reach a size equivalent to a 10-μm square or larger insome cases. It is believed that the fluorescent dye-containing resinparticles undergo precipitation and/or aggregation after a long-termstorage thereof.

Therefore, an object of the present invention is to provide a mediumwhich is capable of inhibiting precipitation and/or aggregation,particularly aggregation, of fluorescent dye-containing resin particlesand enables the fluorescent dye-containing resin particles to be usedfor staining after a long-term storage without having to performcomplicated operations.

Technical Solution

In order to realize at least one of the above-described objects, thepresent invention provides the following medium:

a medium for storing fluorescent dye-containing resin particles,wherein, in a particle-containing liquid obtained by adding fluorescentdye-containing resin particles to the medium, the rate of change in thebackscatter intensity (transmitted light) at the center of the height ofthe particle-containing liquid left to stand for 24 hours after theaddition is not less than −1% based on the particle-containing liquidimmediately after the addition.

Advantageous Effects of Invention

By storing fluorescent dye-containing resin particles in the medium ofthe present invention, cellular tissues can be stained using thefluorescent dye-containing resin particles even after a long-termstorage with only a simple operation such as pipetting (stirring),without requiring pretreatments such as solvent substitution, which isperformed by repeating appropriate times the operations ofcentrifugation, supernatant removal, dilution with a staining solventand redispersion by ultrasonication, and subsequent filtering treatment,before the use for staining as in conventional technologies.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the staining result obtained in Example 5 usingstreptavidin-modified fluorescent dye-containing resin particlesimmediately after the synthesis thereof.

FIG. 2 shows the staining result obtained in Example 5 usingstreptavidin-modified fluorescent dye-containing resin particles afterone month of storage in the medium of the present invention.

FIG. 3 shows the staining result obtained in Example 11 usingstreptavidin-modified fluorescent dye-containing resin particlesimmediately after the synthesis thereof.

FIG. 4 shows the staining result obtained in Example 11 usingstreptavidin-modified fluorescent dye-containing resin particles afterone month of storage in the medium of the present invention.

FIG. 5 shows the staining result obtained in Comparative Example 7 usingstreptavidin-modified fluorescent dye-containing resin particlesimmediately after the synthesis thereof.

FIG. 6A shows the staining result obtained in Comparative Example 7using streptavidin-modified fluorescent dye-containing resin particlesafter one month of storage in a medium.

FIG. 6B is a sketch illustrating the positions of coarse aggregates inthe staining result of Comparative Example 7 shown in FIG. 6A.

FIG. 7 is a chart showing the relationship of the average particle sizeof the resulting fluorescent dye-containing resin particles with respectto the amount of the added resin material in the production of thefluorescent dye-containing resin particles used in Examples andComparative Examples.

MODE FOR CARRYING OUT THE INVENTION

The medium according to the present invention will now be describedconcretely.

[Medium]

The medium according to the present invention is:

a medium for storing fluorescent dye-containing resin particles,wherein, in a particle-containing liquid obtained by adding fluorescentdye-containing resin particles to the medium, the rate of change in thebackscatter intensity (transmitted light) at the center of the height ofthe particle-containing liquid left to stand for 24 hours after theaddition is not less than −1% based on the particle-containing liquidimmediately after the addition.

That is, when a particle-containing liquid is prepared by addingfluorescent dye-containing resin particles to the medium of the presentinvention, with the backscatter intensity (transmitted light) measuredat the center of the height of the particle-containing liquidimmediately after the addition being defined as “I₀” and the backscatterintensity (transmitted light) measured at the center of the height ofthe particle-containing liquid that is left to stand for 24 hours afterthe addition being defined as “I₂₄”, the rate of change in thebackscatter intensity (transmitted light) at the center of the height ofthis particle-containing liquid, which is D (%) determined by thefollowing formula:

D=(I ₂₄ −I ₀)/I ₀×100

satisfies the relationship of D≥−1. This rate of change, D, representsthe degree of aggregation of fluorescent dye-containing resin particlesstored in the medium of the present invention and serves as an index forevaluating the performance of the medium in storing fluorescentdye-containing resin particles.

In other words, from a different perspective, whether or not a mediumfor storing fluorescent dye-containing resin particles corresponds tothe medium of the present invention, that is, whether or not a mediumfor storing fluorescent dye-containing resin particles satisfies therelationship of D≥−1, can be verified by an evaluation method comprisingthe following steps (1) to (4):

(1) the step of obtaining a particle-containing liquid by addingfluorescent dye-containing resin particles to the medium of interest;

(2) the step of measuring the backscatter intensity (transmitted light)I₀ at the center of the height of the particle-containing liquidimmediately after the addition;

(3) the step of again measuring the backscatter intensity (transmittedlight) I₂₄ at the center of the height of the particle-containing liquidafter leaving the particle-containing liquid to stand for 24 hours; and

(4) the step of determining whether or not the following requirement issatisfied based on the thus measured I₀ and I₂₄.

(I ₂₄ −I ₀)/I ₀×100≥−1

In the present invention, the “backscatter intensity (transmittedlight)”, based on which the rate of change (D) is determined, refers tothe intensity of a transmitted light or back-scattered light that isgenerated when a light emitted from a light source travels straightwhile transmitting or being repeatedly scattered through a sample.

In the present invention, the reason why the backscatter intensity(transmitted light) is measured at the center of the height of aparticle-containing liquid in as follows.

In cases where fluorescent dye-containing resin particles to be used forpathological staining are stored in a medium, aggregation of theparticles during a long-term storage causes coarse aggregates to begenerated when the pathological staining is performed and thisinterferes with making a correct determination. On the other hand, evenif precipitation of the fluorescent dye-containing resin particlesoccurred during a long-term storage in the medium, by re-dispersing theprecipitated particles, the pathological staining can be performedwithout generation of coarse aggregates. In view of this, for theevaluation of the performance of a medium in storing fluorescentdye-containing resin particles, it is required to be able to observeparticle aggregation separately from particle precipitation.

When a particle-containing liquid is left to stand and the particlesdispersed therein aggregate, since a change occurs in the amount of theback-scattered light throughout the particle-containing liquidregardless of its height position, the backscatter intensity(transmitted light) at the center of the height is reduced accordingly.

Meanwhile, when the dispersed particles simply precipitate, although thebackscatter intensity (transmitted light) at the upper and lower partsof the particle-containing liquid changes with time as the precipitationof the particles proceeds, the backscatter intensity (transmitted light)at the center of the height hardly changes.

Therefore, by setting the position of measuring the backscatterintensity (transmitted light) at the center of the height of aparticle-containing liquid, the degree of aggregation of fluorescentdye-containing resin particles in a medium can be properly evaluated, sothat the performance of the medium in storing the fluorescentdye-containing resin particles can be appropriately evaluated. In themost ideal mode of the medium of the present invention, no aggregationof fluorescent dye-containing resin particles occurs in the medium evenafter a long-term storage and, in this case, D=0.

In the present invention, the value of D is prescribed to be not lessthan −1 because, from the present inventors' experiences, it wasconsidered appropriate to make judgment based on this value whenevaluating the performance of a medium in inhibiting the aggregation offluorescent dye-containing resin particles. This has also be confirmedfrom its relationship with the evaluation results of immunostaining andmorphological staining that were obtained in the below-describedExamples and Comparative Examples using fluorescent dye-containing resinparticles after one month of storage in a medium. It is noted here thatthe present inventors presume that the rate of change in the backscatterintensity (transmitted light) at the center of the height of aparticle-containing liquid left to stand for one month also has acertain correlation with the value of D.

In the medium of the present invention, the value of D may be largerthan 0 (D>0) depending on the measurement conditions; however, such avalue of D presents no problem.

In the present invention, the wavelength of the light to be irradiatedat the center of the height for the measurement of the backscatterintensity (transmitted light) is not necessarily particularly restrictedas long as the backscatter intensity (transmitted light) of theparticle-containing liquid of interest can be appropriately measured inrelation to fluorescent dye-containing resin particles. However, for anappropriate measurement, it is desired that the wavelength of theirradiated light be longer than the particle size of the fluorescentdye-containing resin particles. Here, a light having a wavelength ofabout 880 nm is preferably used since it does not require a specialmeasuring instrument.

Further, in the present invention, the measuring instrument used for theevaluation is also not particularly restricted as long as it is capableof appropriately measuring the backscatter intensity (transmitted light)at the center of the height of the particle-containing liquid ofinterest, and examples of a preferred measuring instrument includeTURBISCAN (trademark) manufactured by Formulaction SA. According to thismeasuring instrument, it is also possible to measure the backscatterintensity (transmitted light) while changing the height position. Thisis, however, not to forbid use of other spectrophotometer in themeasurement of the backscatter intensity (transmitted light).

(Constituents)

In the medium of the present invention, as described above, the rate ofchange (D (%)) satisfies the specific range prescribed in the presentinvention. The concrete constitution of the medium of the presentinvention that satisfies such a rate of change (D (%)) varies dependingon the type, the condition of surface modification and the like of thefluorescent dye-containing resin particles to be stored and is thus notuniformly and strictly specified here; however, the medium of thepresent invention typically comprises a buffer, a protein and asurfactant.

Protein

The protein that can constitute the medium of the present invention isnot particularly restricted as long as it does not impair the functionsof fluorescent dye-containing resin particles and is capable ofinhibiting aggregation of fluorescent dye-containing resin particles.However, when the medium of the present invention is used for storingfluorescent dye-containing resin particles to be used for pathologicalstaining, it is desired that the protein be capable of inhibitingnon-specific adsorption to the cellular tissue to be stained.Accordingly, examples of a preferred protein include those proteins thatare generally used as a blocking agent, such as BSA and casein.

The content of the protein in the medium of the present invention can beadjusted as appropriate within a range where aggregation of fluorescentdye-containing resin particles can be inhibited; however, it is desiredto be, for example, 10% by weight or less (e.g., in a range of 1 to 10%by weight) with respect to the whole medium.

Surfactant

The surfactant that can constitute the medium of the present inventionis not particularly restricted as long as it does not impair thefunctions of fluorescent dye-containing resin particles and is capableof inhibiting aggregation of fluorescent dye-containing resin particles.However, when the medium of the present invention is used for storingfluorescent dye-containing resin particles to be used for pathologicalstaining, there are cases where the fluorescent dye-containing resinparticles are directly used for pathological staining in a state ofbeing diluted with the medium of the present invention. In the cellulartissues, parts where the cell nucleus is located are negatively chargedbecause of the phosphate residues constituting the nucleic acid, whereasthose parts other than the cell nucleus tend to be positively charged.Therefore, in order to minimize the non-specific adsorption to thecellular tissues, it is desired to use a nonionic surfactant as thesurfactant. Particularly, polyoxyethylene sorbitan fatty acid esterssuch as Tween (registered trademark)-based surfactants can be preferablyused and, thereamong, Tween (registered trademark) 20 can beparticularly preferably used.

The content of the surfactant in the medium of the present invention canbe adjusted as appropriate within a range where aggregation offluorescent dye-containing resin particles can be inhibited; however, itis desired to be, for example, in a range of 0.1% by weight or less withrespect to the whole medium.

Buffer

The buffer that can constitute the medium of the present invention isnot particularly restricted as long as it does not impair the functionsof fluorescent dye-containing resin particles, and a variety ofconventionally known buffers can be used.

In a preferred mode of the present invention, the medium of the presentinvention is used for storing fluorescent dye-containing resin particlesto be used for pathological staining. In this case, as the fluorescentdye-containing resin particles used for pathological staining, thereactive functional group-containing fluorescent dye-containing resinparticles described below in the section of “Mode of FluorescentDye-containing Resin Particles”, particularly those comprising amolecule that is likely to form a bond based on affinity interactionsuch as biotin, streptavidin and avidin, are often employed. Therefore,it is preferred that the buffer used in the present invention have a pHin a range that does not cause degeneration of such a molecule. Further,in pathological staining with such fluorescent dye-containing resinparticles, since the fluorescent dye-containing resin particles may besubjected to the staining in a state of being diluted with the medium ofthe present invention, it is preferred that the buffer have a pH in arange that is suitable for pathological staining. From thesestandpoints, the buffer used in the present invention preferably has apH in a range of 6.0 to 8.0, more preferably in a range of 6.9 to 7.6.Examples of a preferred buffer type include phosphate-bufferedphysiological saline (PBS), Tris-HCl buffer, phosphate buffer (excludingPBS), and a combination of two or more of these buffers.

Other Components

In the medium of the present invention, in addition to the buffer,protein and surfactant, other component(s) such as a preservative mayalso be incorporated, as long as the functions of fluorescentdye-containing resin particles are not impaired and aggregation offluorescent dye-containing resin particles can be inhibited. Examples ofthe preservative include sodium azide (NaN₃).

It is desired that the preservative be incorporated in the buffer at aconcentration of 0.015 N or less.

Production Method

The medium of the present invention can be obtained by dissolving theprotein and the surfactant as well as the “other component(s)”, whichis/are optionally added, in the buffer in accordance with a conventionalmethod.

The combination and composition ratio of the constituents, namely theprotein and the surfactant, and the “other component(s)” that is/areoptionally added vary depending on the type and the like of thefluorescent dye-containing resin particles to be stored and are,therefore, not uniformly and strictly specified here. However, for theadjustment of the combination and composition ratio, reference can bemade to the results of the below-described Examples and ComparativeExamples.

In the aggregation of fluorescent dye-containing resin particles, it isbelieved that the electrostatic relation between the fluorescentdye-containing resin particles and/or the electrostatic relation betweenthe medium and the fluorescent dye-containing resin particles are alsoinvolved. Accordingly, for the determination of the combination andcomposition ratio of the constituents, reference can also be made to thezeta potential of the fluorescent dye-containing resin particles in themedium. The zeta potential of the fluorescent dye-containing resinparticles can be measured using a common zeta potential-measuring device(such as “Zetasizer Nano” manufactured by Malvern Instruments Ltd.) andadjusted with the protein and preservative as well as, depending on thecase, the surfactant. For example, when the medium of the presentinvention is set to contain a buffer of pH 6.0 to 8.0, the medium of thepresent invention can be prepared by adjusting the combination andcomposition ratio of the constituents and/or finely adjusting the pH ofthe buffer within a range of 6.0 to 8.0 such that the fluorescentdye-containing resin particles have a zeta potential of 0 mV to −10 mVin the resulting medium of the present invention.

(Fluorescent Dye-Containing Resin Particles to be Stored)

The term “fluorescent dye-containing resin particles to be stored” usingthe medium of the present invention refers to a substance having astructure in which plural fluorescent dye molecules are immobilized in astate of being encapsulated in a resin particle by chemical or physicalaction, and the form thereof is not particularly restricted.

Examples of the fluorescent dye-containing resin particles of interestin the present invention include conventionally known fluorescentdye-containing resin particles, and their resin may be composed of athermosetting resin such as a melamine resin or a thermoplastic resinsuch as a polystyrene resin. However, when the fluorescentdye-containing resin particles are used for pathological staining,clearing with an organic solvent such as xylene may be performed in theprocess of pathological staining. Therefore, from the standpoint ofinhibiting elution of the fluorescent dye in the clearing step using anorganic solvent such as xylene, fluorescent dye-containing resinparticles whose resin is composed of a thermosetting resin capable ofimmobilizing a fluorescent dye inside its fine cross-linked structure,such as a melamine resin, are preferred.

The size of the fluorescent dye-containing resin particles is notparticularly restricted as long as it is suitable for the intendedapplication such as immunostaining of a tissue section; however, it isusually 10 nm to 500 nm, preferably 40 nm to 200 nm, more preferably 50nm to 200 nm. Further, the variation coefficient, which represents thevariation in the particle size, is also not particularly restricted;however, it is usually 20% or less, preferably 5 to 15%. Fluorescentdye-containing resin particles having such a particle size can beobtained by, for example, the below-described production method.

The size of a fluorescent dye-containing resin particle can bedetermined by taking an electron micrograph thereof using a scanningelectron microscope (SEM), measuring the cross-sectional area of thefluorescent dye-containing resin particle and then determining theparticle size as the diameter of a circular area corresponding to themeasured value (area-equivalent circle diameter). With regard to theaverage particle size (average particle diameter) and the variationcoefficient of a group of fluorescent dye-containing resin particles,after measuring the particle size (particle diameter) for a sufficientnumber (for example, 1,000) of the fluorescent dye-containing resinparticles in the above-described manner, the average particle size iscalculated as the arithmetic mean of the measured values and thevariation coefficient is calculated by the following equation:100×(standard deviation of particle size)/(average particle size).

Fluorescent Dye

The fluorescent dye constituting the fluorescent dye-containing resinparticles to which the present invention is applied is not particularlyrestricted and may be a conventionally known fluorescent dye.

Fluorescent dyes that are generally available or can be prepared may beclassified into, for example, rhodamine-based dye molecules,squarylium-based dye molecules, cyanine-based dye molecules, aromatichydrocarbon-based dye molecules, oxazine-based dye molecules,carbopyronine-based dye molecules, pyrromethene-based dye molecules,Alexa Fluor (registered trademark, manufactured by Invitrogen)-based dyemolecules, BODIPY (registered trademark, manufactured byInvitrogen)-based dye molecules, Cy (registered trademark, manufacturedby GE Healthcare)-based dye molecules, DY (registered trademark,manufactured by Dyomics GmbH)-based dye molecules, HiLyte (registeredtrademark, manufactured by AnaSpec Inc.)-based dye molecules, DyLight(registered trademark, manufactured by Thermo Fisher ScientificK.K.)-based dye molecules, ATTO (registered trademark, manufactured byATTO-TEC GmbH)-based dye molecules, and MFP (registered trademark,manufactured by Mobitec Co., Ltd.)-based dye molecules. The genericnames of these dye molecules are designated based on the main structure(skeleton) or registered trademark of the respective compounds;therefore, those of ordinary skill in the art should be able to properlyunderstand the scope of the fluorescent dyes belonging to the respectivegeneric names without having to bear undue trial and error. It is notedhere thatN,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxdiimideused in the below-described Examples corresponds to an aromatichydrocarbon-based dye molecule.

Further, the fluorescent dye may be subjected to a solubilizationtreatment for the purposes of, for example, improving the emissionintensity of the fluorescent dye and increasing the Stokes shift. Thissolubilization treatment is not particularly restricted as long as it isa technique capable of solubilizing the fluorescent dye, that is,improving the solubility of the fluorescent dye in water. Specificexamples of the solubilization treatment include methods in which afluorescent dye is treated and allowed to react with an acid (e.g.,concentrated sulfuric acid, concentrated hydrochloric acid, acetic acidor formic acid) or an aldehyde (e.g., formaldehyde or acetaldehyde),among which an acid treatment is preferred since it generally shows anexcellent effect.

Further, the emission wavelength of the fluorescent dye can be selectedas desired in accordance with the intended application. For example, inpathological diagnosis, when such an application where staining witheosin or the like for morphological observation and immunostaining witha fluorescent dye are simultaneously performed is postulated, it ispreferred that the fluorescent dye have an emission wavelength in theinfrared to near-infrared range so that the light emitted from thefluorescent dye can be visually observed and the emission wavelength ofthe fluorescent dye does not overlap with that of fluorescence-emittingeosin. For example, a fluorescent dye having its maximum excitationwavelength in a range of 555 to 620 nm and maximum emission wavelengthin a range of 580 to 770 nm is preferred.

Resin

The resin constituting the fluorescent dye-containing resin particles towhich the present invention is applied may be a thermosetting resin or athermoplastic resin. For example, from the standpoint of inhibitingelution of the fluorescent dye in the clearing step using an organicsolvent such as xylene, a resin comprising a thermosetting resin such asa melamine resin, which is capable of immobilizing the fluorescent dyeinside its fine cross-linked structure, is preferred. In a preferredmode of the present invention, the resin constituting the fluorescentdye-containing resin particles to which the present invention is appliedis a thermosetting resin, more specifically a resin consisting of only athermosetting resin such as a melamine resin.

Examples of the thermosetting resin include those which contain astructural unit formed from at least one monomer selected from the groupconsisting of melamine, urea, guanamines (including benzoguanamine,acetoguanamine and the like), phenols (including phenol, cresol, xylenoland the like), xylene and derivatives thereof. Any one of these monomersmay be used individually, or two or more thereof may be used incombination. If desired, one or more co-monomers other than thecompounds may also be used in combination.

Specific examples of the thermosetting resin includemelamine-formaldehyde resins, urea-formaldehyde resins,benzoguanamine-formaldehyde resins, phenol-formaldehyde resins andmetaxylene-formaldehyde resins.

As a starting material of these thermosetting resins, in addition to theabove-described monomers per se, a prepolymer obtained by allowing sucha monomer to react with formaldehyde and other compound such as across-linking agent in advance can also be used. For example, in theproduction of a melamine-formaldehyde resin, generally, methylolmelamine prepared by condensation between melamine and formaldehydeunder an alkaline condition is used as a prepolymer, and this compoundmay further be subjected to alkyl-etherification. Examples of thealkyl-etherification of methylol melamine include methylation forimprovement of the stability in water, and butylation for improvement ofthe solubility in an organic solvent.

Further, in the thermosetting resin, at least some of the hydrogenscontained in the structural unit may be substituted with a chargedsubstituent or a substituent capable of forming a covalent bond. Such athermosetting resin can be synthesized by using, as a starting material,a monomer in which at least one hydrogen is substituted with thesubstituent (derivatized monomer) by a known method. Normally, melamineresins, urea resins, benzoguanamine resins and the like naturallycontain an amino group or a cation generated from a moiety originatedfrom an amino group, and phenol resins, xylene resins and the likenaturally contain a hydroxyl group or an anion generated from a moietyoriginated from a hydroxyl group.

Such a thermosetting resin can be synthesized in accordance with a knownmethod. For example, a melamine-formaldehyde resin can be synthesized byheating and polycondensing methylol melamine prepared in advance in theabove-described manner, with an addition of, as required, a reactionaccelerator such as an acid.

Meanwhile, examples of the thermoplastic resin include those whichcontain a structural unit formed from at least one monofunctionalmonomer selected from the group consisting of styrene, (meth)acrylicacid, alkyl esters thereof, acrylonitrile and derivatives thereof (amonomer having one group involved in polymerization reaction in onemolecule, which group is a vinyl group in the above-described case). Anyone of these monomers may be used individually, or two or more thereofmay be used in combination. If desired, one or more co-monomers otherthan the compounds may also be used in combination.

Specific examples of the thermoplastic resin include polystyrenes,styrene-based resins composed of styrene and other monomer(s),polymethyl methacrylates, acrylic resins composed of (meth)acrylic acid,an alkyl ester thereof and other monomer(s), polyacrylonitriles, ASresins (acrylonitrile-styrene copolymers) and ASA resins(acrylonitrile-styrene-methyl acrylate copolymers), andacrylonitrile-based resins composed of acrylonitrile and othermonomer(s).

The thermoplastic resin may also contain, for example, a structural unitformed from a polyfunctional monomer such as divinylbenzene (a monomerhaving two or more groups involved in polymerization reaction in onemolecule, which groups are vinyl groups in the above-described case),that is, a cross-linked moiety. Examples of such a thermoplastic resininclude cross-linked polymethyl methacrylates.

In the thermoplastic resin, at least some of the hydrogens contained inthe structural unit may be substituted with a charged substituent or asubstituent capable of forming a covalent bond. Such a thermoplasticresin can be synthesized by using, as a starting material, a monomer inwhich at least one hydrogen is substituted with the substituent(derivatized monomer), such as 4-aminostyrene.

Further, the thermoplastic resin may also contain a structural unitcomprising a functional group used for surface modification of theresulting fluorescent dye-containing resin particles. For example, byusing an epoxy group-containing monomer such as glycidyl methacrylate asa starting material, fluorescent dye-containing resin particles on whichepoxy groups are oriented on the surface can be prepared. These epoxygroups can be converted into amino groups by allowing them to react withan excess amount of aqueous ammonia. In the thus formed amino groups,various biomolecules can be incorporated in accordance with a knownmethod. As required, the incorporation of various biomolecules into theamino groups can be carried out through molecules that serve as linkers.

Mode of Fluorescent Dye-Containing Resin Particles

The fluorescent dye-containing resin particles to which the presentinvention is applied comprise the fluorescent dye and resin and may besubjected to surface modification.

The medium of the present invention can be particularly preferably usedfor storing fluorescent dye-containing resin particles that are used forpathological staining such as immunostaining. It is preferred that thefluorescent dye-containing resin particles to which the presentinvention is applied further comprise a reactive functional group sothat the fluorescent dye-containing resin particles are easily boundwith a molecule-recognizing substance (e.g., an antibody) capable ofrecognizing a biological substance to be detected by pathologicalstaining (more specifically, a biological substance that can be anantigen). Examples of the reactive functional group include chemicalfunctional groups, such as a carboxyl group, an amino group, an aldehydegroup, a thiol group and a maleimide group; and molecules that arelikely to form a bond based on affinity interaction, such as biotin,streptavidin and avidin. In the fluorescent dye-containing resinparticles, a linker or spacer having an appropriate chain length mayexist between the main part of the fluorescent dye-containing resinparticles (that is, the part of the fluorescent dye-containing resinparticles that excludes the reactive functional group and optionallinker or spacer).

Method of Producing Fluorescent Dye-Containing Resin Particles

The fluorescent dye-containing resin particles to which the presentinvention is applied can be produced in accordance with a polymerizationstep known for various resins.

Fluorescent dye-containing resin particles whose resin is composed of athermosetting resin can be produced in accordance with a known emulsionpolymerization method. For example, the polymerization step of thefluorescent dye-containing resin particles whose resin is composed of athermosetting resin may be a step of generating fluorescentdye-encapsulating resin particles by heating a reaction mixture whichcontains a fluorescent dye and a resin material (a monomer, an oligomeror a prepolymer), and preferably an appropriate known surfactant and anappropriate known polymerization reaction accelerator, and therebyallowing polymerization reaction of the resin to proceed. In this case,the order of adding the components contained in the reaction mixture isnot particularly restricted.

The conditions of the polymerization reaction (e.g., temperature andtime) can be set as appropriate taking into consideration the type ofthe resin, the composition of the material mixture and the like. For thesynthesis of a thermosetting resin such as a melamine resin, thereaction temperature is usually 70 to 200° C. and the reaction time isusually 20 to 120 minutes. Here, it is appropriate that the reactiontemperature be a temperature at which the performance of the fluorescentdye is not reduced (within the range of the heat resistant temperature).The heating can be performed in a plurality of steps and, for example,the material mixture may first be allowed to react for a certain time ata relatively low temperature and then heated and allowed to furtherreact for a certain time at a relatively high temperature. After thecompletion of the polymerization reaction, impurities such as unreactedresin material, fluorescent dye and surfactant are removed, and the thusgenerated fluorescent dye-containing resin particles can be recoveredand purified.

Further, in the production of fluorescent dye-containing resin particleswhose resin is composed of a thermosetting resin, after thepolymerization step, as required depending on the intended applicationof the fluorescent dye-containing resin particles, a modification stepcan also be performed as a step of introducing the reactive functionalgroup described above in the section of “Mode of FluorescentDye-containing Resin Particles” to the surface of the fluorescentdye-containing resin particles. The introduction of the reactivefunctional group can be appropriately performed by a conventionalmethod.

Meanwhile, fluorescent dye-containing resin particles whose resin iscomposed of a thermoplastic resin can be produced in the same manner asthe fluorescent dye-containing resin particles whose resin is composedof a thermosetting resin, except that, as a polymerization step, a stepof generating fluorescent dye-encapsulating resin particles by heating areaction mixture which contains a fluorescent dye, a resin material anda polymerization initiator (e.g., benzoyl peroxide orazobis-isobutyronitrile) and thereby allowing polymerization reaction ofthe resin to proceed is performed in accordance with a conventionalmethod.

(Application)

The above-described medium of the present invention can be suitably usedfor storing fluorescent dye-containing resin particles, particularlyfluorescent dye-containing resin particles used for pathologicalstaining. From a different perspective, the method of storingfluorescent dye-containing resin particles can be seen as a method whichcomprises adding the fluorescent dye-containing resin particles to themedium of the present invention. The fluorescent dye-containing resinparticles can be stored usually under refrigeration (for example, at 4to 5° C.).

Specific examples of the pathological staining include immunostaining.

EXAMPLES

Examples and Comparative Examples according to the present inventionwill now be described referring to the drawings.

The fluorescent dye-containing resin particles of Examples andComparative Examples were measured or evaluated by the followingmethods.

(Method of Measuring Average Particle Size of Fluorescent Dye-ContainingResin Particles)

A photograph of the fluorescent dye-containing resin particles ofinterest was taken under a scanning electron microscope (SEM), thecross-sectional area was measured for a sufficient number of particles,and the particle size was determined as the diameter of a circular areacorresponding to the respective measured values. In the below-describedSynthesis Examples, the arithmetic mean of the particle sizes of 1,000particles was defined as the average particle size.

Synthesis Examples 1-1 to 1-7: Preparation of Fluorescent Dye-ContainingResin Particles

As fluorescent dye-containing resin particles of Synthesis Examples 1-1to 1-7, using a conventionally known method, fluorescent dye-containingresin particles A1 to A7 having an average particle size of 40, 60, 80,100, 150, 200 and 250 nm, respectively, were each prepared.

As an example of the method of producing the fluorescent dye-containingresin particles, the method of producing the fluorescent dye-containingresin particles A5 is described below.

Synthesis Example 1-5

By treatingN,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxdiimidewith concentrated sulfuric acid, a sulfo group was introduced to give acorresponding sulfonic acid. This sulfonic acid was converted into acorresponding acid chloride by a conventional method.

After adding 14.4 mg of the thus obtained acid chloride to 22.5 mL ofwater, the resultant was heated at 70° C. for 20 minutes on a hotstirrer and 0.65 g of a melamine resin Nikalac MX-035 (manufactured byNippon Carbide Industries Co., Ltd.) was added thereto, followed byheating of the resulting mixture with stirring for another 5 minutes.Then, 100 μL of formic acid was further added, and the resultant washeated with stirring at 60° C. for 20 minutes and subsequently cooled toroom temperature. Thereafter, the resulting reaction mixture was placedin a centrifugal tube and centrifuged at 12,000 rpm for 20 minutes,followed by removal of the resulting supernatant. The precipitates werewashed with ethanol and water.

Then, 0.1 mg of the thus obtained particles was dispersed in 1.5 mL ofEtOH (ethanol), and 2 μL of aminopropyltrimethoxysilane LS-3150(manufactured by Shin-Etsu Chemical Co., Ltd.) was added thereto. Theresultant was allowed to react for 8 hours so as to perform a surfaceamination treatment.

The thus obtained dye-containing nanoparticles were adjusted with PBS(phosphate-buffered physiological saline) containing 2 mM of EDTA(ethylenediamine tetraacetic acid) to a concentration of 3 nM, and thissolution was mixed with SM (PEG) 12 (manufactured by Thermo FisherScientific K.K.; succinimidyl-[N-maleimidopropionamido)-dodecaethyleneglycol]ester) to a final concentration of 10 mM and allowed to react for1 hour. The thus obtained mixture was centrifuged at 10,000 G for 20minutes, and the resulting supernatant was removed. Then, PBS containing2 mM of EDTA was added to disperse the precipitates, and the resultingdispersion was centrifuged again. The precipitates were washed threetimes by the same procedure to give fluorescent dye-containing resinparticles (fluorescent particles) A5 having a maleimide group at aterminal.

When the particle size was measured for the thus obtained fluorescentdye-containing resin particles A5 under an electron microscope by theabove-described method, the average particle size was found to be 150nm.

Synthesis Examples 1-1 to 1-4, 1-6 and 1-7

Fluorescent dye-containing resin particles A1 to A4, A6 and A7 ofSynthesis Examples 1-1 to 1-4, 1-6 and 1-7, which had different particlesizes from the fluorescent dye-containing resin particles A5 ofSynthesis Example 1-5, were also each synthesized in the same manner asin Synthesis Example 1-5, except that the amount of the resin waschanged as appropriate while maintaining the dye/added resin amount inthe synthesis constant.

For reference, FIG. 7 shows the relationship of the average particlesize of the resulting fluorescent dye-containing resin particles withrespect to the amount of the added resin material (the melamine resin inSynthesis Examples 1-1 to 1-7) in a case where fluorescentdye-containing resin particles are synthesized under the same conditionsas in Synthesis Example 1-5.

It is noted here that, in the following descriptions, in order todistinguish the fluorescent dye-containing resin particles A1 to A7 fromthe below-described streptavidin-modified fluorescent dye-containingresin particles, the fluorescent dye-containing resin particles A1 to A7may be referred to as “maleimide group-modified fluorescentdye-containing resin particles A1 to A7”, respectively, and these resinparticles may be collectively referred to as “maleimide group-modifiedfluorescent dye-containing resin particles”.

Synthesis Examples 2-1 to 2-7: Synthesis of Streptavidin-ModifiedFluorescent Dye-Containing Resin Particles

The maleimide group-modified fluorescent dye-containing resin particlesA1 to A7 were each modified with streptavidin in the below-describedmanner to give streptavidin-modified fluorescent dye-containing resinparticles S1 to S7, respectively.

A thiol group addition treatment was performed for streptavidin(manufactured by Wako Pure Chemical Industries, Ltd.), by allowing thestreptavidin to react with N-succinimidyl-S-acetylthioacetate (SATA),and then subjecting the resultant to a known hydroxylamine treatment fordeprotection of S-acetyl group. Then, by filtering the resultant througha gel filtration column, a solution of streptavidin capable of bindingto fluorescent dye-containing resin particles was obtained.

The thus obtained streptavidin solution was mixed with 1 mL of a liquidcontaining fluorescent dye-containing resin particles which was obtainedby diluting the maleimide group-modified fluorescent dye-containingresin particles with PBS containing 2 mM of EDTA to a concentration of 1nM, and the resulting mixture was allowed to react at room temperaturefor 1 hour, whereby the fluorescent dye-containing resin particles werebound with streptavidin. The resultant was then centrifuged and washedwith PBS containing 2 mM of EDTA, and only streptavidin-modifiedfluorescent dye-containing resin particles were recovered.

The thus obtained streptavidin-modified fluorescent dye-containing resinparticles were subjected to various evaluations in a state of being oncediluted with 1% BSA-containing PBS buffer.

Examples 1 to 12 and Comparative Examples 1 to 16 (Medium andFluorescent Dye-Containing Resin Particles)

As a medium, a Tris buffer containing 0.6% α-casein, 0.6% β-casein, 3%BSA, 0.1% Tween (registered trademark) 20 and 0.015 N NaN₃ (pH=6.9) wasemployed in Examples 1 to 6 and Comparative Example 1; a PBS buffercontaining 10% BSA, 0.1% Tween (registered trademark) 20 and 0.05 N NaN₃(pH=7.6) was employed in Examples 7 to 12 and Comparative Example 2; aPBS buffer containing 1% BSA (pH=7.2) was employed in ComparativeExamples 3 to 9; and a polymeric surfactant (0.1% DISPERBYK-194: pH=7.0)was employed in Comparative Examples 10 to 16.

Further, in Examples and Comparative Examples, as fluorescentdye-containing resin particles, the streptavidin-modified fluorescentdye-containing resin particles S1 to S7 were each used as shown in Table1 below.

The fluorescent dye-containing resin particles were subjected to thefollowing storage and evaluations using the medium.

(Storage of Fluorescent Dye-Containing Resin Particles)

The respective streptavidin-modified fluorescent dye-containing resinparticles contained in a 1% BSA/PBS solution were subjected to removalof supernatant, substitution with the medium and then a filteringtreatment (0.65 μm, manufactured by Merck Millipore Corporation).Thereafter, the resultant was adjusted by dilution with the medium to anintended concentration of the streptavidin-modified fluorescentdye-containing resin particles (0.2 nM), thereby giving a mediumcontaining the respective fluorescent dye-containing resin particles.

The fluorescent dye-containing resin particles were stored in this formof being contained in the medium in a refrigerator at 4° C.

(Evaluation of Precipitation and Aggregation of FluorescentDye-Containing Resin Particles)

Precipitation and aggregation of fluorescent dye-containing resinparticles were evaluated using TURBISCAN (trademark) (TURBISCAN Lab)manufactured by Formulaction SA.

Specifically, for the respective streptavidin-modified fluorescentdye-containing resin particles of immediately after the synthesis, amedium containing the fluorescent dye-containing resin particles wasprepared in accordance with the method described above in the section of“Storage of Fluorescent Dye-containing Resin Particles” and, for thismedium containing the fluorescent dye-containing resin particles, thebackscatter intensity (transmitted light) was measured by TURBISCANusing a light source emitting an infrared radiation of 880 nm inwavelength. The measurement was continued for 24 hours while sampling at30-minute intervals.

With the backscatter intensity (transmitted light) measured at thecenter of the height immediately after the start of the measurement(which corresponds to the “backscatter intensity (transmitted light)measured at the center of the height . . . immediately after theaddition”) being defined as “I′₀” and the backscatter intensity(transmitted light) measured at the center of the height after allowingthe medium containing the fluorescent dye-containing resin particles tostand for 24 hours after the start of the measurement being defined as“I′₂₄”, the rate of change in the backscatter intensity (transmittedlight) at the center of the height, D′ (%), was calculated as follows.

D′=(I′ ₂₄ −I′ ₀)/I′ ₀×100

Table 1 shows the rate of change (D′) determined for the respectiveExamples and Comparative Examples. For example, in Example 5, based onthe start of the measurement, the backscatter intensity (transmittedlight) showed a −0.9% change at 24 hours after the start of themeasurement.

(Staining with Fluorescent Dye-Containing Resin Particles)

In order to evaluate the performance of each medium, using therespective streptavidin-modified fluorescent dye-containing resinparticles immediately after the synthesis and after one month of storagein the medium, the following immunostaining, morphological staining andobservation were performed.

As a tissue cell slide, a breast cancer tissue array manufactured by USBiomax, Inc. (model: BR243 Series (24-core); core diameter=1.5 mm) wasemployed.

Immunostaining

The tissue cell slide was deparaffinized in accordance with aconventional method and then washed by substitution with water. The thuswashed tissue cell slide was subjected to a 5-minute autoclave treatmentat 121° C. in 10 mM citrate buffer (pH 6.0), thereby performing anantigen activation treatment.

After the activation treatment, the tissue cell slide was washed withPBS buffer and then subjected to a 1-hour blocking treatment with 1%BSA-containing PBS buffer in a moist chamber. After the blockingtreatment, an anti-HER2 rabbit monoclonal antibody (4B5, manufactured byVentana Medical Systems, Inc.) diluted with 1% BSA-containing PBS bufferto a concentration of 0.05 nM was allowed to react with the tissue cellslide for 2 hours. After washing this tissue cell slide with PBS buffer,the tissue cell slide was further allowed to react for 30 minutes with abiotin-labeled anti-rabbit monoclonal antibody that would bind to 4B5and had been diluted with 1% BSA-containing PBS buffer to aconcentration of 2 μg/mL.

After the reaction with the biotin-labeled anti-rabbit monoclonalantibody, the tissue cell slide was stained with the fluorescentdye-containing resin particles of interest.

It is noted here that, for staining with the fluorescent dye-containingresin particles immediately after the synthesis thereof, the tissue cellslide was allowed to react for 3 hours with the fluorescentdye-containing resin particles of immediately after the synthesis thathad been diluted with 1% BSA-containing PBS buffer to a concentration of0.2 nM, in a neutral pH environment (pH 6.9 to 7.4) at room temperature.Prior to the dilution of the fluorescent dye-containing resin particlesto a concentration of 0.2 nM, the solvent was substituted with themedium by repeating appropriate times the operations of centrifugation,removal of supernatant, dilution with the medium and redispersion byultrasonication, and the resultant was subsequently subjected to afiltering treatment (0.65 μm, manufactured by Merck MilliporeCorporation).

Meanwhile, staining with the fluorescent dye-containing resin particlesafter one month of storage in the medium was also performed in the samemanner, except that the fluorescent dye-containing resin particles thathad been stored for one month in the medium were used in place of thefluorescent dye-containing resin particles of immediately after thesynthesis that were diluted to 0.2 nM. In this case, the fluorescentdye-containing resin particles that had been stored in the form of theabove-described medium containing the fluorescent dye-containing resinparticles were subjected to pipetting (stirring) and then directly usedfor staining without being diluted. In Examples 1 and 7, however, thefluorescent dye-containing resin particles were directly used forstaining without being subjected to pipetting since no precipitationthereof was observed. In the present specification, unless otherwisespecified, the term “pipetting” means stirring of a liquid of interestthat is performed by repeating the operations of sucking up anddischarging the liquid using a pipette.

In any of the above-described cases, after the reaction with thefluorescent dye-containing resin particles, the tissue cell slide waswashed with PBS buffer.

Morphological Staining

The tissue cell slides subjected to the immunostaining were each furthersubjected to morphological staining.

Specifically, the immunostained tissue cell slide was subjected tohematoxylin staining (HE staining) for 1 minute using Mayer'shematoxylin solution. Then, the tissue cell slide was washed withrunning water of about 45° C. for 3 minutes. Next, an operation ofimmersing the tissue cell slide in pure ethanol for 5 minutes wasrepeated four times to perform washing and dehydration. Subsequently, anoperation of immersing the tissue cell slide in xylene for 5 minutes wasrepeated four times to perform clearing. Lastly, the tissue section wasmounted with a mounting medium (“Entellan New”, manufactured by MerckKGaA) to give a sample slide for observation.

Observation

The tissue section on the sample slide that had been subjected to theimmunostaining and morphological staining was allowed to emitfluorescence by irradiating thereto a prescribed excitation light. Thetissue section in this state was observed and photographed under afluorescence microscope (BX-53, manufactured by Olympus Corporation). Itis noted here that the observation and photographing were performed in10 visual fields for each core (a single tissue spot) on the sampleslide. In this process, an objective lens of ×40 magnification and anocular lens of ×10 magnification were used. Further, the bright spotswere measured by ImageJ FindMaxima method.

The excitation light was set to have a wavelength of 575 to 600 nmthrough an optical filter. In addition, the wavelength range (nm) of thefluorescence to be observed was also set at 612 to 682 nm through anoptical filter.

The conditions of the excitation wavelength in the microscopeobservation and image acquisition were set such that the intensity ofthe irradiation light in the vicinity of the center of the visual fieldwas 900 W/cm² for excitation at 580 nm. In the image acquisitionprocess, a photograph was taken by arbitrarily setting the exposure timesuch that the image brightness was not saturated (for example, theexposure time was set at 4,000 μs).

The evaluation results are shown in Table 1 below. For reference, FIGS.1, 3 and 5 show the stained images obtained using the fluorescentdye-containing resin particles immediately after the synthesis inExamples 5 and 11 and Comparative Example 7, respectively; and FIGS. 2,4 and 6A show the stained images obtained using the fluorescentdye-containing resin particles after one month of storage in Examples 5and 11 and Comparative Example 7, respectively.

As for the determination of the presence or absence of coarseaggregates, an evaluation of “x” (presence of coarse aggregates) wasgiven when about 10 visual fields were observed for each tissue cellslide and three or more aggregates having an apparent size of 1 to 2-mmsquare (that is, an actual size equivalent to 2.5 to 5-μm square) orlarger were observed under a microscope. For example, in the case ofComparative Example 7, on the stained image shown in FIG. 6A which wasobtained using the fluorescent dye-containing resin particles after onemonth of storage, three coarse aggregates were confirmed as illustratedin the sketch of FIG. 6B.

TABLE 1 Evaluation of storage performance Used fluorescent After 24hours dye-containing Rate of change in the After one resin particleStaining results backscatter intensity month of storage Particle sizeimmediately Composition of (transmitted light) at Staining Pre-stainingType (nm) after synthesis^(†) medium the center of the height, D′ (%)results^(‡) operation Note Example 1 S1 40 ∘ (0.6% α-casein + −0.1 ∘none Example 2 S2 60 ∘ 0.6% β-casein + −0.2 ∘ pipetting Example 3 S3 80∘ 3% BSA + 0.1% −0.4 ∘ pipetting Example 4 S4 100 ∘ Tween 20 + 0.015N−0.8 ∘ pipetting Example 5 S5 150 ∘ NaN₃)/Tris buffer −0.9 ∘ pipettingExample 6 S6 200 ∘ −1 ∘ pipetting Comparative Example 1 S7 250 x −2 xpipetting *1 Example 7 S1 40 ∘ (10% BSA + 0.1% 0 ∘ none Example 8 S2 60∘ Tween 20 + 0.05N 0.1 ∘ pipetting Example 9 S3 80 ∘ NaN₃)/PBS buffer0.3 ∘ pipetting Example 10 S4 100 ∘ 0.5 ∘ pipetting Example 11 S5 150 ∘0.8 ∘ pipetting Example 12 S6 200 ∘ −0.9 ∘ pipetting Comparative Example2 S7 250 ∘ −2.4 x pipetting *1 Comparative Example 3 S1 40 ∘ 1% BSA/PBS(prior −1.2 Δ pipetting *1 Comparative Example 4 S2 60 ∘ art) −1.4 Δpipetting *1 Comparative Example 5 S3 80 ∘ −1.8 Δ pipetting *1Comparative Example 6 S4 100 ∘ −2.4 Δ pipetting *1 Comparative Example 7S5 150 ∘ −2.6 Δ pipetting *1 Comparative Example 8 S6 200 ∘ −3.2 Δpipetting *1 Comparative Example 9 S7 250 x −4.1 xx pipetting *2Comparative Example 10 S1 40 x Polymeric −1.3 xx pipetting *2Comparative Example 11 S2 60 x surfactant −1.5 xx pipetting *2Comparative Example 12 S3 80 x (0.1% −1.9 xx pipetting *2 ComparativeExample 13 S4 100 x DISPERBYK-194) −2.4 xx pipetting *2 ComparativeExample 14 S5 150 x −2.8 xx pipetting *2 Comparative Example 15 S6 200 x−3.5 xx pipetting *2 Comparative Example 16 S7 250 x −5.5 xx pipetting*2 <^(†) Evaluation of staining immediately after synthesis> ∘: Thetissue cell slide was stained. x: The tissue cell slide could not bestained. <^(‡) Evaluation of staining after one month of storage> ∘: Incomparison to the staining performed immediately after the synthesis,80% of the bright spots were maintained. Δ: At least 80% of the brightspots were maintained; however, coarse aggregates were observed. x: Notmore than 80% of the bright spots were maintained and coarse aggregateswere observed. xx: The tissue cell slide could not be stained and coarseaggregates were observed. <Note> *1: Stainable even after one month whenultrasonication, solvent substitution and filtering treatment wereperformed prior to the staining. *2: Not stainable even whenultrasonication, solvent substitution and filtering treatment wereperformed prior to the staining.

1. A medium for storing fluorescent dye-containing resin particles,wherein, in a particle-containing liquid obtained by adding fluorescentdye-containing resin particles to said medium, the rate of change in thebackscatter intensity (transmitted light) at the center of the height ofsaid particle-containing liquid left to stand for 24 hours after saidaddition is not less than −1% based on said particle-containing liquidimmediately after said addition.
 2. The medium according to claim 1,comprising a buffer, a protein and a surfactant.
 3. The medium accordingto claim 2, wherein said surfactant is a nonionic surfactant.
 4. Themedium according to claim 1, wherein said fluorescent dye-containingresin particles have a particle size of 40 nm to 200 nm.
 5. The mediumaccording to claim 1, wherein the wavelength of a light irradiated atsaid center of said height is longer than the particle size of saidfluorescent dye-containing resin particles.
 6. The medium according toclaim 1, wherein said fluorescent dye-containing resin particles areused for pathological staining.
 7. The medium according to claim 1,wherein said fluorescent dye-containing resin particles further comprisea reactive functional group.
 8. The medium according to claim 1, whereina resin constituting said fluorescent dye-containing resin particles isa thermosetting resin.
 9. The liquid of claim 2, wherein the protein iscasein.
 10. The liquid of claim 2, wherein the protein comprises 0.6%α-casein, 0.6% β-casein, and 3% BSA.
 11. The liquid of claim 2, whereinthe protein comprises 10% BSA.